Fire safety systems include, among other things, detection devices and notification devices. Detection devices include smoke, heat or gas detectors that identify a potentially unsafe condition in a building or other facility. Detection devices can also include manually operated pull stations. Notification devices, often referred to as notification appliances, are devices that provide an audible and/or visible notification of an unsafe condition, such as a “fire alarm”.
In its simplest form, a fire safety system may be a residential “smoke alarm” that detects the presence of smoke and provides an audible alarm responsive to the detection of smoke. Such a smoke alarm device serves as both a detection device and a notification appliance.
In commercial, industrial, and multiple-unit residential buildings, fire safety systems are more sophisticated. In general, a commercial fire safety system will including one or more fire control panels that serve as distributed control elements. Each fire control panel may be connected to a plurality of distributed detection devices and/or a plurality of distributed notification appliances. The fire control panel serves as a focal point for “problem” indicating signals generated by the distributed detection devices, and as source of activation (i.e. notification) signals for the distributed notification appliances. Most fire safety systems in larger buildings include multiple fire control panels connected by a data network. The fire control panels employ this network to distribute information regarding alarms and maintenance amongst each other. In such a way, notification of a fire or other emergency may be propagated throughout a large facility.
Moreover, centralized control of large safety systems may be accomplished by a dedicated or multi-purpose computing device, such as a personal computer. The computer control station is typically configured to communicate with the multiple fire control panels via the data network.
Using this general architecture, fire safety systems are scalable to accommodate various factors including the building layout, the needs of the building management organization, and the needs of the users of the building. To achieve scalability and flexibility, fire safety systems may include any number of computer control stations, remote access devices, database management systems, multiple networks of control panels, and literally hundreds of detection and notification devices. Fire safety systems may further incorporate and/or interact with security systems, elevator control systems, sprinkler systems, and heating, ventilation and air conditioning (“HVAC”) systems.
One of the many sources of costs in fire safety systems is the wiring and material costs associated with the notification appliances. Building safety codes define the specification for notification appliance wiring, voltage and current. For example, according to building safety codes, notification appliances are intended to operate from a nominal 24 volt signal which provides the power for the notification appliance to perform its notification function. For example, an alarm bell, a strobe light, or an electronic audible alarm device operates from a nominal 24 volt supply. In general, however, notification devices are required to operate at voltages as low as 16 volts. The delivery of power to the distributed notification appliances requires a significant amount of wiring and/or a significant number of distributed power sources.
In particular, notification appliances are typically connected in parallel in what is known as a notification appliance circuit or NAC. Each NAC is connected to a power source, such as a 24 volt source, and includes a positive conductor, a ground conductor, and multiple notification appliances connected across the two conductors. The power source may be disposed in a fire control panel or other panel. The positive and ground NAC conductors serve to deliver the operating voltage from the 24 volt power source, to the distributed notification appliances. Because the positive and ground conductors have a finite conductance, i.e. they have impedance, there is a practical limit to how long an NAC may extend from the power source before the voltage available across the NAC conductors falls below the required operating voltage. For example, if copper wire conductors having 0.02 ohms/ft are used, then 100 feet of wire conductors will exhibit 2.0 ohms of resistance. If the power draw through the conductors is 2 amps, then there is a four volt drop of voltage over the 100 feet of wire. The same wire will produce an eight volt drop over 200 feet of wire, which will typically provide too little voltage to devices at the end of the loop.
It is noted that increasing the current draw on the NAC also increases the voltage drop on the NAC because it increases the voltage drop over the resistive conductors. Accordingly, the number of devices on a particular NAC, as well as the length of the NAC, are limited, at least for a given source voltage.
In addition, the power source must be able to provide power to all the NACs in the absence of mains electrical power. Accordingly, while the 24 volt power source of an NAC may ordinary be obtained via conversion of the mains AC electrical power, a battery back-up is also required. In the prior art, two 12-volt batteries have been employed as the secondary power source. Thus, a panel that provides power to an NAC generally requires a source of 24 volts converted from the mains AC electrical power, as well as a battery back-up.
The limitations on NAC physical length and NAC device capacity are exacerbated by this need for the battery backup power. NAC physical length as discussed herein means the length of the power and ground conductors from the power source of the NAC. In general, the actual voltage of the battery backup power source varies from 20.4 to 26.0 volts during the useful life of the batteries. Building code standards require that the NAC be operation throughout the useful life of the battery, and thus when the battery output voltage is as low as 20.4 volts. When the low output source voltage is combined with the voltage drop over the power conductors of the NAC, the ability of the NAC to deliver adequate voltage over long conductor lengths is severely hampered.
To address the limitations of NAC due to voltage drop, extending the coverage of notification appliances often requires increasing the number of power sources. To this end, special powered appliance circuit extension devices may be employed. These powered extension devices are panels that connected to an existing fire control panel and emulate a notification appliance or device. However, the powered extension device provides NAC powered signals to additional NACs. Thus, the powered extension device has its own power source and battery backup power source to power its own NACs. These NACs operate as extensions of the NAC of the fire control panel to which the powered extension device is connected. The use of the powered extension devices effectively extends the coverage that may be achieved with a single fire control panel. The powered extension device is less costly to implement than a fire control panel, but never the less requires additional equipment and battery costs.
One way to extend NAC coverage without adding fire control panels and powered extension devices is to select lower resistance power conductors. For example, a switch from 18 gauge wire to the thicker 14 gauge wire can greatly extend the acceptable length (and/or device capacity) of on NAC. However, thicker wires have significantly higher costs due to the quantity of copper in thicker wires.
Accordingly, there exists a need to reduce costs in notification appliance circuits that arise from the need to provide sufficient voltage and power to notification appliances distributed throughout a building or facility.